Already known from prior art is a three-way catalytic converter which can clean three major toxic components (HC, CO and NO.sub.x) in the exhaust gas. The operation of the three-way catalytic converter is effectively attained when the exhaust gas is in a state wherein excess air as well as excess fuel is not left in the exhaust gas, i.e., the air-fuel ratio of the exhaust gas (the ratio of air remaining in the exhaust gas to fuel remaining in the exhaust gas) is maintained near the stoichiometric value.
For maintaining the air-fuel ratio near the stoichiometric value, an air injection system has been provided in the prior art for controlling the amount of secondary air introduced into the exhaust manifold in accordance with electrical signals transmitted from an oxygen-concentration-cell type air-fuel sensor, for example, an O.sub.2 sensor which is arranged in an exhaust pipe of the engine. This air injection system has a vacuum-operated flow control valve which includes a spring-urged diaphragm which forms a vacuum chamber on one side thereof. To this chamber, a vacuum signal is selectively introduced from the intake manifold of the engine in accordance with the electrical signals of the O.sub.2 sensor. Thus, a vacuum force, which is increased or decreased in accordance with the electrical signals, is generated on the diaphragm for controlling the amount of the secondary air directed to the exhaust manifold.
However, the above described known system exhibits a drawback wherein the rate of increase in the vaccum force of the diaphragm is not maintained at a constant value during all engine load conditions, since the vacuum level in the intake manifold of the engine decreases in accordance with an increase in the load of the engine, i.e., an increase in the opening of a throttle valve of the engine. Thus, the rate of increase in the vacuum force of the diaphragm of the flow control valve occurring during the high load condition is lower than the rate of increase occurring during the low load condition. Consequently, the speed of change in the amount of the secondary air during the high-load condition becomes low. Therefore, due to the above reasons, an ideal air-fuel ratio control cannot be carried out, thereby causing an unsatisfactory operation of the three-way catalytic converter to occur.